Well, that is just poor handling of the situation by the European Union. I still believe that it will be difficult for these people, to obtain arms and ammunition to conduct an attack on European targets. That said, a much more possible scenario is some of these new ISIS members joining the already existing Jihadi cells in Europe.
I did not suggest they will attack in human waves, WW1 style as many liberal politicians currently think about granting voting rights to all foreigners, they wont need to initially. Europe will be crushed anyway.
I am not sure, you understand, that war doesnt have to be waged by weapons, its simply process during which you want the other party to accept your will.
I do not see those economic migrants as enemies by the way. They are simply tool in the hands of western upper class, that serves to curb the rights of natives. Tactics, that was before also used in Africa and Americas for financial gain during the process known as colonization.
It is not plausible anymore to think, that this is merely "poor handling" of situation by officials. Situation has been going on for years like this, too many people and money are involved for it to be all purely coincidental.